To provide an intrusion detecting device capable of generating one of different warnings in proper timing dependent on the behavior of an intruder entering an alert region to thereby browbeat the intruder effectively, the intrusion detecting device (1) includes a plurality of sensor units (8 and 9), a sound generating device (29) capable of generating first and second warnings different from each other one at a time, and a warning generating unit (23). The sensor units (8 and 9) have a plurality of alert areas (A1 to A4) defined from a site distant from and to another site near to a subject to be monitored for security purpose and operable to detect an intruder H entering any of the alert areas (A1 to A4). The warning generating unit (23) is operable to cause the sound generating device (29) to issue the first warning when detection is made by the sensor unit (8), and also to cause the sound generating device (29) to issue the second warning when detection is made by at least the sensor unit (9) during a first predetermined length of time T1 subsequent to the detection by such sensor unit (8).